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[libvirt] QMP Capability Detection Issues with GIT version of QEMU

I ran into trouble today trying to use the GIT level of QEMU.

In a nutshell: the capability detection with QMP is failing
and the fallback using -help isn't working with the GIT level
probably due to help text reformatting.

The failure reason is that QEMU cannot bind to the
QMP monitor socket in the /var/lib/libvirt/qemu directory.
That's because the child process is stripped of all
capabilities and this directory is chown'ed to qemu:qemu
by the QEMU driver.

Note that this is failing with the release QEMU as well,
with the difference that the fallback is working there.

I am willing to provide a patch, however I'd like
to get feedback on the approach to use:

1. Add back Linux capabilities CAP_DAC_OVERRIDE,
   CAP_DAC_READ_SEARCH to allow QEMU to bind to
   the monitor socket.
   This seems to be hacky/dodging the problem.

2. Use a separate directory for the QMP probing
   instance of QEMU.

3. Run the QMP QEMU under the configured qemu
   user. This would be my favorite.

4. Other ideas?


Mit freundlichen Grüßen/Kind Regards
   Viktor Mihajlovski

IBM Deutschland Research & Development GmbH
Vorsitzender des Aufsichtsrats: Martin Jetter
Geschäftsführung: Dirk Wittkopp
Sitz der Gesellschaft: Böblingen
Registergericht: Amtsgericht Stuttgart, HRB 243294

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